"Retroreflective" is defined in ASTM E 284-81a, Standard Definitions of Terms Relating to Appearance of Materials, as "reflection characterized by the flux in an incident beam being returned in the direction close to the direction from which it came, this effect occurring over a wide range of angles."
At the present time, retroreflective articles are prepared by applying transparent or translucent glass spheres to plastic film. Another plastic film or multiple films are then laminated over the spheres using heat and pressure, thus trapping and encapsulating the spheres in a controlled fashion. One side of the article is then metalized, or it may have been metalized prior to the encapsulating step, so as to reflect light. Light waves passed through the spheres are reflected by the specular metalized surface back into the spheres. The spheres refract the light waves so as to enhance and intensify the image received by the human eye. In some cases, other shapes than spheres may be used but in all cases a physical particle inclusion in the article is necessary to cause the amount of refraction so as to obtain a retroreflective characteristic.
While such retroreflective materials provide an excellent degree of retroreflectivity, the cost is typically five to ten times the cost of nonretroreflective articles that are commonly used commercially.
While the physical particle inclusion types of articles are accepted by the Department of Transportation and other agencies to meet their standards, materials for other applications where increased visibility is desired, but not necessarily of the standards required by various governmental agencies, are not available because of the prohibitive cost.
For example, point of purchase, signage or advertising media, which could include some degree of retroflectivity, need not be engineered to meet government specifications for retroreflectivity.
In order to understand the instant invention, reference to three other definitions in ASTM E 284-81a is made. These are:
"Specular reflection" is defined as "reflection without diffusion, in accordance with the laws of optical reflection, as in a mirror."
"Diffuse reflection" is defined as "reflection in which flux is scattered in many directions by diffusion at or below the surface."
"Diffusion" is defined as "change of the angular distribution of a beam of radiant flux by a transmitting material or reflecting surface such that the flux incident in one direction is continuously distributed in many directions, the process not conforming (on a macroscopic scale) to the laws of Fresnell (regular) reflection and refraction and there being no change in frequency (wave length) of the monochromatic component of the flux."